Why is Blender slowing down?
The first thing that comes to mind is that Blender might get bogged down by the number of steps you have performed which it stores in RAM, in order for you to Undo your actions.
The default value for this is 32 steps, and you can change this in your Preferences:

As you can see, you can also set a memory limit to these steps.
There is no paid or pro version of Blender. There is no bad programming to pester people who don't pay for this software, there is no planned obsolescence - it's free and open source.